Thin films of Group III nitride, such as gallium nitride (GaN), are used in the production of efficient optoelectronic light emitters. Conventionally, GaN has been grown directly on sapphire substrates (Al2O3). The GaN is grown in thin layers as opposed to in a single three-dimensional growth mode in order to achieve a high quality crystalline structure of the epitaxial growth. Growing the epitaxial layers of GaN on silicon as opposed to on sapphire offers considerable cost savings because of the economies of scale from the large production of silicon for the semiconductor industry. A large amount of equipment for the production of crystalline silicon has already been depreciated. That equipment can now be used in the production of light emitting diodes (LEDs).
Current attempts to grow high quality epitaxial layers of GaN on silicon substrates, however, have not been entirely successful. Because of the great difference between the lattice constants and thermal expansion coefficients of GaN and silicon, GaN is not well suited for epitaxial growth directly on a silicon substrate. GaN epitaxial layers often crack upon cooling to room temperature because even at growth temperatures above 1000° C. the lattice constant of GaN is much smaller than that of crystalline silicon. In addition, GaN has a much larger coefficient of thermal expansion than does silicon. So as the layers of GaN grown on silicon at high temperature cool to room temperature, the smaller lattice distance of the GaN crystals relative to the silicon crystals becomes even more pronounced. The GaN layers deposited directly on silicon are subjected to even more tensile stress as they cool and can even cause the underlying silicon substrate to bow.
Consequently, attempts have been made to grow buffer layers between the silicon substrate and the epitaxial GaN layers in order to compensate for the differing lattice constants and thermal expansion coefficients of GaN and silicon. For example, buffer layers of AlN, AlGaN and AlGaIN have been grown between the silicon substrate and the GaN layers. Growing the buffer layers is a costly manufacturing step not only because of the materials used in the buffer layers but also because of the additional time required to grow the buffer layers. A sufficient number of buffer layers each having a sufficient thickness must be grown in order to build up sufficient compressive stress to offset the tensile stress exerted on the GaN crystal lattice by the larger silicon crystal lattice.
Moreover, the quality of the epitaxial GaN layers that can be grown over existing buffer layers has been poor. Current methods of forming buffer layers of AlN and AlGaN have resulted in epitaxial growth of GaN layers that contain structural defects such as discontinuities, dislocations and faults. These defects degrade the morphology and optical properties of the GaN layers, rendering the GaN layers unsuitable for use in high quality LEDs.
A method is sought for growing buffer layers over a silicon substrate faster and more cheaply than currently practiced but yet that allows high quality epitaxial GaN layers with fewer structural defects to be grown over the buffer layers.